It is well known that microcircuits are intricate and delicate articles whose performance depends on maintaining the functions provided by extremely small circuit features. Reliable operation depends on protecting the entire microcircuit from environmentally caused harm arising from sources such as mechanical damage, corrosion caused by moisture and airborne chemical vapors, and particulate contamination. This invention provides for lower manufacturing cost of printed circuit board assemblies using a known type of structure for protecting individual microcircuits from damage arising from environmental sources after manufacturing is done. The invention does not compromise either the level of protection or the operating characteristics.
Microcircuits are most frequently formed on a silicon wafer by photolithographic processes. Typically there will be hundreds of individual microcircuits formed on the wafer by a series of photolithographic steps. After the photolithographic process steps are completed and the microcircuits have been formed, the wafer is sliced by a diamond saw to form a number of small rectangular dice, each carrying (usually) a single microcircuit. In the manufacturing process, small connection pads around the periphery of the actual circuit are formed as a part of the microcircuit's conductors, and these are present on the individual dice. Thin wires are bonded to these connection pads and to other pads off the die itself for connecting the microcircuit to other devices which provide signals and power to the microcircuit, or use the signals which the microcircuit generates. As is well known of course, microcircuits are usually mounted on and electrically connected to a printed circuit board. Depending on its type there will be as many as several hundred individual connections from a microcircuit to the circuit board on which it is mounted.
The need to protect individual microcircuits from external damage was mentioned above. This protection can be provided by enclosing the microcircuit within a package of some type. When such a package is used, it will have external pins or pads of some kind which are internally connected to the microcircuit, and by which the microcircuit makes its external connections. These connections can take a variety of forms. Pins or legs extending from the package surface can be connected to printed circuit wiring either by soldering or by mounting in a permanent socket mounted on the printed circuit board. External connection pads are usually placed around the outer edge of the package; these also can either be soldered to properly positioned pads on the printed circuit board, or the entire package can be inserted in a socket which has spring contacts which press against the pads. A socket is used where it may be desirable to remove the microcircuit from the circuit board at a later time.
While pre-packaging microcircuits in individual packages during the initial manufacturing process makes them easy to ship and mount, there are some disadvantages. In the first place at least one extra connection for each microcircuit pad is required. Secondly, additional area on a printed circuit board must be allotted for each microcircuit package, which is a problem if space constraints are important. Thirdly, the additional steps in the mounting process may add to the cost. That is, other things being equal, the microcircuit mounting part of the process involves additional steps in mounting the individual microcircuit in the package, and then mounting the package itself on the printed circuit board. Each of these process steps is likely to add cost and reduce yield/reliability. Eliminating any of these steps has the potential to reduce cost and increase yield. Accordingly, there has been motivation to develop a means of microcircuit connection which omits as a part of the microcircuit manufacturing process, the step of packaging individual microcircuits.
This alternate type of microcircuit connection has been realized in the process known as chip-on-board (COB) surface mounting. Briefly, in this process a die cut conventionally from a wafer and carrying a completed microcircuit with exposed connection pads is mechanically attached to a printed circuit board, typically with an adhesive. Then leads are attached between the microcircuit's connection pads and the corresponding pads on the circuit board. Lastly, some type of mechanical protection for the microcircuit and the leads is applied. The COB process as presently practiced mounts a microcircuit in perhaps 30-50% less area on the printed circuit board than the same microcircuit when mounted as pre-packaged component. This is an important advantage because the application may not permit a larger printed circuit board and in any case, a smaller board is cheaper.
There is a definitive discussion of the COB process in the publication ANSI/IPC-SM-784, Guidelines for Chip-on-Board Technology Implementation, dated November 1990, and available from the Institute for Interconnecting and Packaging Electronic Circuits, 7380 N. Lincoln Ave., Lincolnwood, Ill. 60646 (hereafter "the Guidelines"). The remainder of this disclosure assumes the reader to be familiar with the Guidelines.
The printed circuit board used in COB mounting may be made either from an inorganic material such as a ceramic, or from an epoxy or other organic material. Because organic printed circuit boards are cheaper, it is preferred to use them unless some special factor requires use of inorganic printed circuit boards. Of the organic materials used for printed circuit boards, cellulose epoxy mat (CEM) is among the least expensive, but is also among the most sensitive to heat. A designation for a type of generic CEM board material is CEM-1 . 100% epoxy mat is a more expensive board material that has advantages where conductor paths on both sides are necessary. FR-4 is a generic term for 100% epoxy mat. Because of its low price and generally good performance, CEM is usually preferred if compatible with the manufacturing processes required and the usage expected. CEM and 100% epoxy mat boards have a foil layer from which are etched the conductors which electrically connect the components carried on the board. Most often, the foil layer is formed of copper because of its outstanding electrical and heat conductivity. Areas of the foil conductors form connection pads to which the wires from the microcircuits are attached.
The Guidelines describe three different connection processes for electrically connecting the die's connection pads to the printed circuit board pads. These are thermocompression, thermosonic, and ultrasonic wire bonding. These processes rely on at least one of pressure, heat and vibration to form a bond between individual wires and pads. The Guidelines explain these processes and their advantages and disadvantages. Briefly, which of these processes to use in a particular situation depends on a number of factors, such as the number of interconnections to be made, the reliability required, the type of use to which the printed circuit board will be put, the density of the microcircuits on the board, etc.
Ultrasonic bonding, also called wedge bonding, is often preferred because it is overall the cheapest, under the proper conditions it makes a very satisfactory connection, and it does not rely on external heating of the parts. Wedge bonding uses a wedge-shaped bonding tool to press the wire strongly against the pad. High frequency acoustic energy without external heat is then applied to the bonding tool which vibrates the wire against the pad to form a mechanical and electrical bond between the wire and the pad. Aluminum wire is customary for use when wedge bonding as usually forming both cheaper and better bonds than gold, the other wire commonly used for chip connection.
Wedge bonding of aluminum wire to the aluminum die pads formed during the photolithographic steps of microcircuit manufacture is a widely accepted technique. The pads on the printed circuit board to which the die pads are electrically connected are part of the copper foil which is etched to form the printed circuit conductor paths on the board. It is known to be quite difficult to consistently form acceptable bonds between aluminum wire and copper pads. For this reason, the Guidelines, p. 11, specify that the copper foil on the printed circuit board must have a gold over nickel coating to create a surface to which the aluminum wire will form an acceptable bond. Even though most of the copper foil and the gold layer on it is removed during the etching process, it is customary to plate the nickel and gold layers on the foil before it is attached to the printed circuit board. The use of the nickel and gold layers allows a good electrical and mechanical connection between the aluminum wire and the copper foil, but the cost of the gold plating on the copper foil is a disadvantage of COB mounting.
I am not familiar with the metallurgical factors which influence those skilled in art to apply the gold and nickel layers to the copper foil before the bonding step. I suspect that yields may suffer and even that longevity of the bond may decrease when the gold and nickel layers are not present. Transactions on Components, Hybrids, and Manufacturing Technology, vol. CHMT-7, No. 4, December, 1984, Effects of Ambient Atmosphere on Aluminum-Copper Wirebond Reliability, by Dennis R. Olsen et al. discusses a mechanism by which the wedge bonded interface between an aluminum wire and a bare copper surface deteriorates over time in a vacuum, but appears to have a relatively long life in the presence of air. The received wisdom with respect to wedge bonding of aluminum wires to the copper pads on printed circuit boards is that a gold layer must be present on the copper surface for high yield and a long-lived bond.
Since the conductor pattern on a printed circuit board is formed by etching a copper foil which is attached to the substrate insulating sheet, this means that the gold layer must be originally applied to the entire printed circuit board surface. It is possible to recover at least some of the gold from the etchant but these steps add cost to the process. In any case, gold remains on the exterior surface of the entire conductor pattern, creating added material cost of the final printed circuit board. Accordingly, there is economic motivation to omit the gold plating on copper surfaces preparatory for wedge bonding aluminum wires to them if bond quality does not suffer.
Mechanical protection of the microcircuit and the connections to it is usually provided by encapsulating or enclosing the entire die and surrounding board with a self-hardening liquid, typically either silicone or epoxy, see the Guidelines, p. 33. Such encapsulation provides excellent protection against mechanical damage and particulate contamination. And while these materials are not completely hermetic, they do limit substantially the amount of air and water vapor which can reach the individual connections between the printed circuit board and the circuitry on the die itself.
To summarize, the apparent present state of the art regarding the processes used for bonding aluminum wire to a copper surface such as in the COB chip mounting process, is to plate a nickel layer on the copper surface, plate a gold layer on the nickel layer, and then wedge bond the aluminum wire to the gold layer surface.